<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .float{
            width: 200px;
            height: 200px;
            background-color: lightcyan;
            float: left;
        }

        .box{
            width: 100%;
            height: 200px;
            background-color: lightgreen;
            display: flex;
        }
    </style>
</head>
<body>
    <div class="float"></div>
    <div class = 'box'></div>
</body>
</html>